The present invention relates to a method for setting, in particular for variably setting, a feedback power of a fundamental frequency clocked converter on the power supply side with a bridge circuit equipped with controllable semiconductor switches, and to an apparatus provided for implementing the method.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
A converter device or a converter circuit can include at least one power supply connection and phase sections which are provided therefor as power supply connection and function as current paths, in other words three phases in the case of connection to a three-phase power supply, then the actual converter comprising the bridge circuit that implements the conversion functionality, and a DC link at the output of the converter, wherein a first output of the converter forms a first DC link terminal and a second output of the converter forms a second DC link terminal. A DC link voltage is present between these two DC link terminals during operation, said DC link voltage arising as a DC voltage due to the conversion. For the purpose of buffering and smoothing the DC link voltage, a DC link capacitance is usually arranged between the DC link terminals.
In the case of a positive power flow arising during operation into the DC link, the converter operates approximately like a so-called B6 diode bridge, since only the diodes that the converter comprises carry the current and transistors that are respectively reverse-connected in parallel therewith as semiconductor switches are deenergized. Conversely, in the case of power flow from the DC link into the power supply, the diodes are reverse-biased and precisely one path into the power supply is made possible for the current by corresponding activation of individual transistors at each point in time. One preferred circuit for the bridge circuit forming the converter is a so-called IGBT B6 bridge (insulated gate bipolar transistor), which comprises a passive parallel diode bridge that enables purely passive operation in one power direction, that is to say e.g. for the power flow into the DC link, and an active feed-in/feedback power flow, that is to say correspondingly e.g. from the DC link into the power supply. In fundamental frequency operation, switching edges for driving the transistors, which are referred to hereinafter generally as semiconductor switches, are at so-called natural triggering instants, that is to say intersection points of the power supply voltages which are fed to the circuit or to the device via the individual current paths, that is to say—in the case of a three-phase power supply, the individual lines—designated hereinafter by R, S, T—of the three-phase power supply with the power supply voltages UR, US and UT fed by means thereof.
Fundamental frequency clocked operation of converters is distinguished by some advantages that make it expedient to use. Since the semiconductor switches, which can also be referred to as current valves, are not clocked at high frequency, this results for example in low switching losses and furthermore in the possibility of using simple and cost-effective commutation inductors and power supply filters, and finally in low excitation of system oscillations that can lead e.g. to high motor bearing currents. Finally, the operation of converters of this type is generally robust with respect to power supply disturbances, power supply unbalances and the like.
A considerable disadvantage of this operating mode hitherto has been, for example, that a relatively large reactive power requirement arises in the case of no load or in the case of weak loading of the converter. This leads to a loading of the power supply system for the user and to increased energy costs. The technology thereby becomes more difficult to impart or market. A further disadvantage is that, in the case of power supply voltage dips, the DC link voltage follows the power supply voltage and energy stored in the DC link for buffering is thus lost.
A simple measure for avoiding these disadvantages is to switch on the semiconductor switches only when energy feedback into the power supply is necessary. A simple criterion for this may be e.g. the rising of the DC link voltage above a threshold value. The feedback, that is to say the driving of the semiconductor switches, can be deactivated again as soon as e.g. DC link voltage has fallen below a lower threshold value or the active current into the power supply has fallen on average below a specific value.
In order to achieve a stable operating state and a sufficient effect of this measure, the two threshold values must not be too close together. Moreover, the power supply voltage and thus the natural rectification voltage may be subject to large fluctuations, for which the thresholds must likewise be sufficiently spaced apart. Moreover, the DC link functioning as an energy buffer can be utilized all the better in the event of load changes, the greater the separation between the two thresholds. During operation, however, a rapid change in the DC link voltage can have adverse effects on the process to be operated/the application to be operated and lead e.g. to a reduction of the torque accuracy in the case of electric drives and hence deviations from a desired position.
To date, the problem outlined above has been solved e.g. either by using a large commutation inductance with the disadvantages of increased costs and an undesirable increased voltage drop, or by using a large DC link capacitance with the disadvantages of increased costs, or by means of a control-technological compensation of the voltage changes in the DC link and likewise the disadvantages of increased costs owing to requirements made of a short controller clock cycle or on account of additionally required circuitry measures.
In one conventional converter, a changeover is made between fundamental frequency uncontrolled operation and clocked voltage-controlled operation when the operating mode of the converter at a point in time is found to be unsuitable for the instantaneous power requirements made of the converter.
It would therefore be desirable and advantageous to provide an improved method for controlling instances of fundamental frequency feed-in/feedback to obviate prior art shortcomings and to maintain the abovementioned major advantages of occasional turn-off of the transistors T1-T6 during motor feed-in operation (robust behavior of a diode bridge in the event of power supply disturbances, no additional harmonic reactive power requirement in the case of no load and partial load, reduced switching losses) and at the same time avoids rapid and large changes in the DC link voltage at the start of fundamental frequency feedback operation.